Every mental phenomenon is characterized by what the Scholastics of the Middle Ages called the intentional (or mental) inexistence of an object, and what we might call, though not wholly unambiguously, reference to a content, direction towards an object (which is not to be understood here as meaning a thing), or immanent objectivity. Every mental phenomenon includes something as object within itself, although they do not all do so in the same way. In presentation something is presented, in judgement something is affirmed or denied, in love loved, in hate hated, in desire desired and so on. This intentional in-existence is characteristic exclusively of mental phenomena. No physical phenomenon exhibits anything like it. We could, therefore, define mental phenomena by saying that they are those phenomena which contain an object intentionally within themselves.

Brentano coined the expression "intentional inexistence" to indicate the peculiar ontological status of the contents of mental phenomena. According to some interpreters the "in-" of "in-existence" is to be read as locative, i.e. as indicating that "an intended object [...] exists in or has in-existence, existing not externally but in the psychological state," (Jacquette 2004, p. 102), while others are more cautious, affirming that: "It is not clear whether in 1874 this [...] was intended to carry any ontological commitment," (Chrudzimski and Smith 2004, p. 205).

A major problem within intentionality discourse is that participants often fail to make explicit whether or not they use the term to imply concepts such as agency or desire, i.e. whether it involves teleology. Dennett (see below) explicitly invokes teleological concepts in the "intentional stance". However, most philosophers use intentionality to mean something with no teleological import. Thus, a thought of a chair can be about a chair without any implication of an intention or even a belief relating to the chair. For philosophers of language, intentionality is largely an issue of how symbols can have meaning. This lack of clarity may underpin some of the differences of view indicated below.

Other 20th-century philosophers such as Gilbert Ryle and A.J. Ayer were critical of Husserl's concept of intentionality and his many layers of consciousness,[6] Ryle insisting that perceiving is not a process[7] and Ayer that describing one's knowledge is not to describe mental processes.[8] The effect of these positions is that consciousness is so fully intentional that the mental act has been emptied of all content and the idea of pure consciousness is that it is nothing[9] (Sartre also referred to "consciousness" as "nothing").[10]

PlatonistRoderick Chisholm has revived the Brentano thesis through linguistic analysis, distinguishing two parts to Brentano's concept, the ontological aspect and the psychological aspect.[11] Chisholm's writings have attempted to summarize the suitable and unsuitable criteria of the concept since the Scholastics, arriving at a criterion of intentionality identified by the two aspects of Brentano's thesis and defined by the logical properties that distinguish language describing psychological phenomena from language describing non-psychological phenomena.[12] Chisholm's criteria for the intentional use of sentences are: existence independence, truth-value indifference, and referential opacity.[13]

In current artificial intelligence and philosophy of mind, intentionality is sometimes linked with questions of semantic inference with both skeptical and supportive adherents.[14]John Searle argued for this position with the Chinese room thought experiment, according to which no syntactic operations that occurred in a computer would provide it with semantic content.[15] Others are more skeptical of the human ability to make such an assertion, arguing that the kind of intentionality that emerges from self-organizing networks of automata will always be undecidable because it will never be possible to make our subjective introspective experience of intentionality and decision making coincide with our objective observation of the behavior of a self-organizing machine.[16]

Daniel Dennett offers a taxonomy of the current theories about intentionality in Chapter 10 of his book The Intentional Stance. Most, if not all, current theories on intentionality accept Brentano's thesis of the irreducibility of intentional idiom. From this thesis the following positions emerge:

Roderick Chisholm (1956), G.E.M. Anscombe (1957), Peter Geach (1957), and Charles Taylor (1964) all adhere to the former position, namely that intentional idiom is problematic and cannot be integrated with the natural sciences. Members of this category also maintain realism in regard to intentional objects, which may imply some kind of dualism (though this is debatable).

The latter position, which maintains the unity of intentionality with the natural sciences, is further divided into three standpoints:

Proponents of the eliminative materialism, understand intentional idiom, such as "belief", "desire", and the like, to be replaceable either with behavioristic language (e.g. Quine) or with the language of neuroscience (e.g. Churchland).

Holders of realism argue that there is a deeper fact of the matter to both translation and belief attribution. In other words, manuals for translating one language into another cannot be set up in different yet behaviorally identical ways and ontologically there are intentional objects. Famously, Fodor has attempted to ground such realist claims about intentionality in a language of thought. Dennett comments on this issue, Fodor "attempt[s] to make these irreducible realities acceptable to the physical sciences by grounding them (somehow) in the 'syntax' of a system of physically realized mental representations" (Dennett 1987, 345).

Those who adhere to the so-called Quinean double standard (namely that ontologically there is nothing intentional, but that the language of intentionality is indispensable), accept Quine's thesis of the indeterminacy of radical translation and its implications, while the other positions so far mentioned do not. As Quine puts it, indeterminacy of radical translation is the thesis that "manuals for translating one language into another can be set up in divergent ways, all compatible with the totality of speech dispositions, yet incompatible with one another" (Quine 1960, 27). Quine (1960) and Wilfrid Sellars (1958) both comment on this intermediary position. One such implication would be that there is, in principle, no deeper fact of the matter that could settle two interpretative strategies on what belief to attribute to a physical system. In other words, the behavior (including speech dispositions) of any physical system, in theory, could be interpreted by two different predictive strategies and both would be equally warranted in their belief attribution. This category can be seen to be a medial position between the realists and the eliminativists since it attempts to blend attributes of both into a theory of intentionality. Dennett, for example, argues in True Believers (1981) that intentional idiom (or "folk psychology") is a predictive strategy and if such a strategy successfully and voluminously predicts the actions of a physical system, then that physical system can be said to have those beliefs attributed to it. Dennett calls this predictive strategy the intentional stance.

They are further divided into two theses:

adherence to the Normative Principle

adherence to the Projective Principle

Advocates of the former, the Normative Principle, argue that attributions of intentional idioms to physical systems should be the propositional attitudes that the physical system ought to have in those circumstances (Dennett 1987, 342). However, exponents of this view are still further divided into those who make an Assumption of Rationality and those who adhere to the Principle of Charity. Dennett (1969, 1971, 1975), Cherniak (1981, 1986), and the more recent work of Putnam (1983) recommend the Assumption of Rationality, which unsurprisingly assumes that the physical system in question is rational. Donald Davidson (1967, 1973, 1974, 1985) and Lewis (1974) defend the Principle of Charity.

The latter is advocated by Grandy (1973) and Stich (1980, 1981, 1983, 1984), who maintain that attributions of intentional idioms to any physical system (e.g. humans, artifacts, non-human animals, etc.) should be the propositional attitude (e.g. "belief", "desire", etc.) that one would suppose one would have in the same circumstances (Dennett 1987, 343).

Working on the intentionality of vision, belief, and knowledge, Pierre Le Morvan (2005)[17] has distinguished between three basic kinds of intentionality that he dubs "transparent", "translucent", and "opaque" respectively. The threefold distinction may be explained as follows. Let's call the "intendum" what an intentional state is about, and the "intender" the subject who is in the intentional state. An intentional state is transparent if it satisfies the following two conditions: (i) it is genuinely relational in that it entails the existence of not just the intender but the intendum as well, and (ii) substitutivity of identicals applies to the intendum (i.e. if the intentional state is about a, and a = b, then the intentional state is about b as well). An intentional state is translucent if it satisfies (i) but not (ii). An intentional state is opaque if it satisfies neither (i) nor (ii).

The claim that all mental states are intentional is called intentionalism, the contrary being anti-intentionalism.

Some anti-intentionalism, such as that of Ned Block, is based on the argument that phenomenal conscious experience or qualia is also a vital component of consciousness, and that it is not intentional. (The latter claim is itself disputed by Michael Tye.)[18]

Another form of anti-intentionalism associated with John Searle regards phenomenality itself as the "mark of the mental" and sidelines intentionality.[19]

A further form argues that some unusual states of consciousness are non-intentional, although an individual might live a lifetime without experiencing them. Robert K.C. Forman argues that some of the unusual states of consciousness typical of mystical experience are Pure Consciousness Events in which awareness exists, but has no object, is not awareness "of" anything.

Several authors have attempted to construct philosophical models describing how intentionality relates to the human capacity to be self-conscious. Cedric Evans contributed greatly to the discussion with his "The Subject of Self-Consciousness" in 1970. He centered his model on the idea that executive attention need not be propositional in form.[20]